This invention relates generally to R-F receivers and more particularly to multioctave, high-resolution, super-heterodyne receivers utilizing harmonic mixing and mixed-base coding techniques to resolve ambiguities caused by time-coincident signals.
It is often necessary, in the pursuit of electronic intelligence and electronic countermeasure activities, to detect signals which may occur anywhere within a wide range of the R-F spectrum. In the past, a wide variety of approaches has been taken to resolve this problem, the four most common approaches being:
(1) The channelized receiver;
(2) The double-conversion, channelized, supper-heterodyne receiver;
(3) The instantaneous frequency measurement (IFM) receiver; and
(4) The microscan receiver. Each will now be described.
The channelized receiver represents the ideal receiver if performance is the only consideration; however; to cover the radar frequency range of 0.5 to 20 GHz and provide xc2x15 MHz resolution would require 1,950 filters plus amplifiers, an inordinately large, costly unit.
The second approach, the double-conversion, channelized, super-heteodyne receiver, prefilters the RF band into convenient sub-bands, each of which is then heterodyned into a common first IF amplifier. The IF output is further channelized by a second set of band filters following the first IF. These filtered outputs are again heterodyned into a second IF amplifier and finally channelized by a set of contiguous bandpass filters. The received signal frequency is determined by decoding the video detector outputs of each channel. Several design problems are inherent in this approach, among which are: a) measuring the frequencies of time-coincident signals, b) resolving the ambiguity caused by a signal entering the receiver at the many subband filter crosscovers, and c) the complexity, large size and expense of the equipment.
The third approach is the instantaneous frequency measurement receiver. Normally, to cover the RF bandwidth, a channelized super-heterodyne is used to reduce the bandwidth to an octave or less. This octave bandwidth is then processed by one or more frequency discriminators that cover the band. Each discriminator consists of two delay lines and a phase detector, the output of the phase detector and subband channel detector then being processed to provide the digitally encoded frequency word. In addition to its size, cost, and complexity, the IFM receiver has the same problems as the double-conversion channelized super-heterodyne receiver, as described above.
A fourth approach is the microscan receiver, the heart of which consists of a dispersive delay line that is in the signal path. These lines result in a relatively narrow bandwidth and high insertion loss; therefore, a channelized super-heterodyne receiver, with all its drawbacks, is required prior to the microscan processing. In operation, the signal whose frequency is to be determined is mixed with a VCO that is rapidly tuned over the band of interest in a time less than the minimum radar pulse width whose frequency is to be measured. The heterodyned IF signal is a chirp (linear FM) signal. After amplification, the signal is recompressed by a dispersive delay line. The time spent in the delay line will be a function of the frequency of the signal. By measuring this time, and knowing in what subband the signal was received, the signal frequency can be determined.
Many problems have been found in implementing this type of receiver. Most of them occur in the delay line and include: a) triple travel; b) insertion loss; c) narrow band width; d) temperature sensitivity; e) slow data rate; and f) all of the problems encountered with a channelized super-heterodyne receiver, as detailed above.
In order to provide the basis for a clear understanding of the present invention, it is helpful at this point to briefly discuss the mixed-base code notation and its relationship to more conventional number systems. The discussion will include basic principles of a simple mixed-base code and of a staggered mixed-base code suitable for use in the apparatus according to the invention.
A variety of different number systems have been employed for coding purposes, the most common being those number systems utilizing a common base. Each digital word is written or coded in shorthand notation as a series of digits an, anxe2x88x921, anxe2x88x922, . . . a0 where the a""s are coefficients of successive powers (from right to left) of an integer termed the base or radixxe2x80x94thus the positional notation of common-base number systems is merely an arrangement wherein each position or column is weighted according to successive powers of the base, and therefore constitutes an abbreviated version of the more complex expression xcexa3Kxe2x88x92akxe2x88x92ra where r is the base (radix) and oxe2x89xa6akxe2x89xa6rxe2x88x921. In the decimal system for example, r=10 so that the coefficients ak (digits forming a word in the positional notation) extend from 0 through 9. Similarly, binary numbers (words) are based on r=z, with ak taking on the values 0 and 1; octal words on r=8, oxe2x89xa6akxe2x89xa67; and so forth.
The mixed-base notation, employed in the present invention, on the other hand, refers to a coding scheme or digital format wherein the digits in each column or position are referenced to a different base, with no positional weighting. The scheme is, in fact, based on the use of different moduli m for each position so that the digit in each column is actually the remainder deriving from the division of a real integer by an integral multiple of the modulo mk for that column or position. In a 3-4-5 mixed-base code, for example, the positional notation is based on modulo 3, modulo 4, and modulo 5, so that the word xe2x80x9c15xe2x80x9d in decimal notation is designated xe2x80x9c030xe2x80x9d in 3-4-5 mixed-base notation. It will readily be observed that this result is a consequence of the fact that 15 is an integral multiple of 3, viz 15/3=5 remainder 0, 15/4=3 remainder 3, and similarly, 15/5=remainder 0. It will also be apparent that digits in each column or position can take on only the values oxe2x89xa6bkxe2x89xa6mkxe2x88x921 where bk is the digit in the position k based on modulo mk.
It will be apparent, therefore, that a simple mixed-base code is limited to a series of nonredundant digital words over a range of values equal to the lowest common denominator of the moduli. In the case of the 3-4-5 mixed-base notation, 3xc3x974xc3x975=60 (the lowest common denominator of 3, 4, and 5) so that decimal numbers 0 through 59 inclusive, for example, may be expressed without ambiguity. Of course, the non-redundant word capacity of a mixed-base code increases as the number of moduli increases or as the value of the integer representing each modulo increases.
Detailed discussion of the mixed-base notation is available in the prior art (see, e.g., U.S. Pat. No. 3,488,594 to J. Caballero, Jr., and U.S. Pat. No. 3,019,975 to R. E. Williams), so that the presentation here has been relatively brief, confined to very basic principles of the code. It is convenient for purposes of the present invention, to depart slightly from the usual mixed-base notation by allowing the digits bk in each position K to take on the values 1xe2x89xa6bkxe2x89xa6mk. In other words, a digit in the position based on modulo 3 may have a value of either 1, 2, or 3, corresponding to the normal values 0, 1, or 2, respectively, with similar considerations applying to digits in positions based on the remaining moduli of the code. Here again, this is purely a matter of convenience, rather than a limitation of the inventive principles.
The development of wideband microwave receivers may be accomplished in a manner representing a practical application of the mixed-base coding concept. For example, a receiver may be designed with one channel for each column (position) in the mixed-base code, the number of parallel outputs from each channel corresponding to the numerical value of the base (modulo integer) of each column. The receiver may be designed in such a manner that, as the frequency of the receiver signal varies from one end of the band to the other, the output of each channel cycles in accordance with the mixed-base code. For a 3-4-5 mixed base code, the receiver would consist of three channels having a total of twelve output leads (i.e., 3+4+5=12). The outputs for each channel could be derived from the respective outputs of narrowband I-F filters, each channel thereby requiring a number of filters equal to the numerical value of the base of the mixed-code column to which that channel corresponds. Thus, for the 3-4-5 code, the first channel would require three filters, the second channel four filters, and the third channel five filters. By using a comb of harmonically related signals as the local oscillators for each channel, the outputs of the filters will cycle in the required manner. In this fashion, receiver resolution equivalent to 60 narrowband channels, or frequency cells, can be obtained by use of only 12 filters and 3 harmonic generators. The AN/SLQ-7 and AN/ULR-12 EMC receivers are typical of equipment using technique.
Caballero, Jr., U.S. Pat. No. 3,488,594 discloses an RF receiver that appears to operate in the same general manner as the receiver disclosed herein, but with several important drawbacks. The primary drawback of the invention of Caballero, Jr., is that it does not disclose and, in fact, teaches away from resolving an ambiguity which arises when two signals are received at the same time. Where it is necessary to determine the individual frequencies detected, as is sometimes required by advanced radar warning systems or ELINT applications to which the present invention is directed, it is absolutely necessary to provide unambiguous, time-coincident, frequency detection.
Another drawback of the Caballero, Jr., invention is that it provides for only 10 MHz resolution. Often-times it is desirable, especially in military applications and where wide portions of the RF spectrum, such as 0.5 to 20 GHz, are to be covered by the receiver, to obtain better resolution. Applicant""s invention overcomes this problem also.
Additionally, the Caballero, Jr., receiver cannot cover more than a single octave of the RF spectrum. This requires many different receivers, each covering a single octave, to provide detection capability throughout the many-octave RF spectrum, particularly 1.0 to 18.16 GHz. Applicant""s invention overcomes this problem through the use of harmonic mixing.
Finally, Caballero, Jr., discloses a receiver which is costly to build. Each channel requires bandpass filters (frequency cells) of different bandwidths to monitor the incoming IF signal from the mixer. These filters are costly and requiring these different filters with different bandwidths adds additional sums to the total expenditure necessary to produce such a receiver. The disclosed invention requires only that the number of filters within each channel be increased. The bandwidth of each filter is always the same and depends upon the desired resolution of the receiver.
Although Caballero, Jr., was an improvement over the prior art, that invention, like all of the previously described receivers, has shortcomings and disadvantages vis-a-vis providing instantaneous unambiguous frequency measurements over the multioctave RF bandwidth, particularly where high-resolution measurements of time-coincident signals are required.
The present invention is a wide-band, high-resolution receiver for concurrently monitoring a plurality of contiguous frequency channels in a preselected band of the RF spectrum and for detecting and indicating the presence of a single signal, or time-coincident signals, within any of said channels by conversion of the signals(s) to a digital mixed-base code notation representative of the RF channel(s) carrying said signal(s). Said receiver is comprised of two or more channels, each of identical elements, though of different values. Each channel includes harmonic mixer means, an IF amplifier, a plurality of filter means and a plurality of detector means, each unique to a filter means. The output of all detector means is fed to a frequency sorter means which provides an output corresponding to the RF channel in which the received signal was transmitted. The use of two channels of the above-described receiver will unambiguously indicate the frequency of one RF signal. The use of three or more channels, as needed, will unambiguously detect and determine the RF frequency of time-coincident signals.
It is therefore an object of the present invention to resolve ambiguities in super-heterodyne receivers caused by time-coincident signals.
A further object of the present invention is to utilize harmonic mixing as a technique for resolving ambiguities in a super-heterodyne receiver.
Another object of the present invention is to provide multioctave bandwidth capability in a super-heterodyne receiver utilized for instantaneous frequency measurements.
A still further object is to provide unambiguous signal detection in a more efficient manner at lower cost and in a smaller size package than existing receivers.